1. Field of the Invention
The invention relates to a sprocket with dog gear comprising a sprocket and a dog gear coaxially, which is exclusively used as the sprocket for a transfer for a four-wheeled vehicle.
2. Description of the Related Art
A sprocket with dog gear is known to be manufactured by a method in which a matrix having no gear tooth is formed by hot forging means and then the respective tooth profiles of the dog gear and the sprocket are cut in the matrix, a method in which the dog gear and the sprocket are separately formed by cutting and then they are combined with each other, or a method in which the matrix having the tooth profile of the dog gear is formed by cold forging means and then the tooth profile of the sprocket is cut in the matrix.
Any conventional method uses cutting means to form the tooth profile of the sprocket and thus has bad productivity, thereby causing an increase in cost.
More particularly, the method in which the sprocket and the dog gear separately formed are combined with each other is provided with a step of combination, thereby causing a further increase in the cost.
Moreover, the following approach is adopted to use the cutting means. When a diameter of the dog gear is larger than that of the sprocket, as illustrated in FIG. 8, a groove 3 is formed in a boundary between a dog gear 1 and a sprocket 2 so as to secure a clearance for a cutting tool in order to avoid interference with the cutting tool, whereby a distance L between an effective portion M of the gear tooth of the sprocket 2 and the dog gear 1 must be increased. Thus, a thickness of the sprocket with dog gear is axially increased.
To use the forging means, it is not necessary to consider the clearance for the cutting tool. Thus, the forging means is suitable for a formation of the sprocket with dog gear with a thickness axially reduced. However, the forging means requires high pressure, thus a heavy load is applied to a die, and consequently the forging means is disadvantageous in the formation of the highly accurate tooth profile.
A forging technique capable of obtaining high accuracy at low pressure and a development of an axially thin product are therefore desired.
The invention provides a sprocket with dog gear comprising a dog gear and a sprocket coaxially, which is suitable for a reduction in axial thickness and makes it feasible to reduce a pressure for a forging process and to increase accuracy of a product. The sprocket with dog gear is integrally formed by the following steps: a first step of upsetting by hot forging means, thereby forming a rough having a shape comprising the sprocket and the dog gear; and a second step of increasing accuracy of the respective tooth profiles of the dog gear and the sprocket by setting the rough in a cold forging apparatus and drawing the rough by the cold forging apparatus.
A sprocket with dog gear comprising a dog gear and a sprocket having a diameter smaller than the diameter of the dog gear is formed by the following steps: a first step of upsetting by hot forging means, thereby forming a rough having a shape comprising the sprocket and the dog gear; a second step of increasing accuracy of the respective tooth profiles of the dog gear and the sprocket by setting the rough in a cold forging apparatus and drawing the rough by the cold forging apparatus; and a third step of forming by cutting means a cut-in groove of a fixed depth perpendicularly to an axial direction in a boundary between the dog gear and the sprocket.
Desirably, in at least the second step, a ring-shaped concave groove is coaxially formed in the dog gear of the rough in a surface close to the sprocket, a margin for drawing is set in the tooth profile of the sprocket, and an incompletely-drawn portion produced by the margin for drawing can be formed in the concave groove.
Moreover, in the third step, the incompletely-drawn portion can be removed when the cut-in groove is formed.
Furthermore, a distance between the sprocket and the dog gear can be shorter than a clearance for a cutting tool required to form the sprocket.
A sprocket with dog gear, in which a ring-shaped concave groove is coaxially formed in one surface of a sprocket and a dog gear having a diameter smaller than the diameter of the sprocket is located on an inner wall of the concave groove so that the dog gear and the sprocket axially overlap each other, is formed by the following steps: a first step of upsetting by hot forging means, thereby forming a rough having a shape comprising the sprocket and the dog gear; and a second step of increasing accuracy of the respective tooth profiles of the dog gear and the sprocket by setting the rough in a cold forging apparatus and drawing the rough by the cold forging apparatus.